As our engine bays are so
crowded, a lot of the over heating we suffer is because the air cannot leave
faster enough. This problem is at it's worse at four wheeling speeds as there is
no ram effect from travelling fast to help vent the hot air out. The problem get
even worse the more that is added under the bonnet area like dual batteries, air
compressors etc. Removing the rubber from under the sump, if it is still there,
will help some but otherwise you should use a cowl bonnet or vents in it. It is
just like the engine, if you have restrictive exhaust, it does not matter how
free you intake side is, it cant flow more then you can exit out the exhaust.
You have to see the exhaust as the under bonnet area, the radiator as the engine
and the air flow from speed, or the fans drawing the air through it, as the
intake.So here is a few ideas
below of others that have come up with solutions to this problem.

I did some aero tests to see what would be
the best place to put them. By sticking strips of wool on the bonnet and then
going for a drive I found that the first half of the bonnet has a negative
pressure area as I had surmised. If you look closely you will see the wool at
the front being pulled forward and even straight up. But as the speed increased
to around 40 - 50 mph, the wool was pulled around and kept flat like the rest of
the bonnet indicating that the air stream was being forced flat at the higher
speed. You can really get bogged down when you start to take into account things
like Laminar flow, Random turbulent flow, Bernoulli's equation and Reynolds
numbers etc. Basically having them in the front 1/3 will make it better at speed
( why Rally cars have them there ), and putting them in the rear 1/3 is better
suited to slow 4wd speeds.

Above is Tozovr's XJ who has fitted
Pontiac Grand Prix vents. These are far cheaper
than the ZJ ones. They work better without their drip pans but it is one way to
have control over where the water goes that comes through the vents. From tests
I have seen you can expect between 15 to 25*F lowering of under hood temperature
depending on speed at the time.

Part Numbers and Costs at the time for
these vents:
#10225885 - Left Side Louver $28.95. #10230097- Left Side Drip Pan $8.00
#10225886 - Right Side Louver $28.95. #10230098 - Right Side Drip Pan $8.00

Here is Jeepzz's with original ZJ ones
fitted to his hood and also some vents on the next one that came off a Nissan
260Z coupe. These have been added more towards the rear which is the safest
position due to the points already mentioned. You can see a
full install of adding these vents here.

These vents are on Markus Kamer's Xj which
he bought off me. Normally on a 100 series Landcruiser sitting on at the
rear most pillar ( D pillar ) as an exit vent for internal cabin pressure. They
are very neat looking.

These photos have fitted by John Wood from Sydney
after getting ideas from the above write up. The front ones are placed
exactly where the the fans throw their air onto the underside of the bonnet.
The part numbers are as follows : Small front vents are (Auto Concepts Aust.
P/L) SAAS #01-11971. Large scoops used in reverse are: SAAS #01-119601.
Here is a diagram he made for me to show placement
for any interested in duplicating the setup. The only thing not shown is
placement of aluminium strap glued to the underside of the bonnet each side
of the scoops to prevent movement. He
has found he can now idle in heavy traffic without even raising the
temperature above 95*C which he couldn't before in temps above 30*C.

Don from Arab, Alabama has fitted the ZJ factory
vents toward the rear of the bonnet in the best location for cooling at low
speeds. He has also managed to include the factory drip trays underneath as
well. The trays can be removed and replaced easily depending on the seasons
and cooling requirements.

But I ended up with going the above route
and fitting an all steel replacement induction hood as it was easier to keep
water and dust out as the rear vent is vertical. At low speeds the hot air was
venting directly from under the bonnet and up the screen but as the speed rose
it drew cold air in. You are looking at a 70*F drop in under temperatures with
this sort of setup. See my write up on the above bonnet
here. I have my air intake picking up from the rear vent too and have a
temperature probe there which shows it is drawing air at speed the same as
outside temperature. That is a big drop from the very heated intake point where
it sucks heated air radiated from the headers and engine etc.